For the purpose of imparting heat resistance, functional strength, and other properties to thermosetting resins, powdery or fibrous reinforcers are used. Particularly, as fibrous reinforcers, fibers such as glass fibers, carbon fibers, ceramic fibers, silicon carbide fibers, metal fibers, and boron fibers; whiskers of metal, ceramic, polymer compounds, etc.; plant fibers such as pulp; synthetic fibers; etc., are generally used. Among plant fibers, microfibrillated cellulose has recently attracted attention.
Microfibrillated cellulose is a general term for fibrous celluloses having a microscale fiber diameter, and pulp as well as various plants, microorganisms, etc. are used as starting materials from which microfibrillated celluloses are derived. Depending on the type of starting materials and the process (e.g., mechanical processing and chemical processing), there are various forms of microfibrillated celluloses. As specific examples of the use of such microfibrillated celluloses as fibrous reinforcers in thermosetting resins, there are known cases in which microfibrillated celluloses are mixed with a phenolic resin or a bisphenol A-type epoxy resin.
For example, an outer plate member and a sliding member obtained by mixing and stirring 20 wt. % of microfibrillated cellulose “Celish” (Daicel Chemical Industries, Ltd.) and 80 wt. % of phenolic resin, and molding the mixture are known (see Patent Documents 1 and 2). The “Celish” is a microfibrillated cellulose having an average fiber diameter of 3 μm or less (0.01 to 3 μm), an average fiber length of 5 to 3,000 μm, and a specific surface area of 50 to 300 m2/g, obtained by vigorously beating and microfabricating cotton linters etc. (starting materials for the microfibrillated cellulose) by the action of high shearing and high impact forces (Patent Document 3).
In addition to “Celish”, a composite obtained by impregnating a nonwoven fabric comprising microfibrillated cellulose having a maximum fiber diameter of 1,500 nm or less with a bisphenol A-type epoxy resin, and thermally curing the resultant by a hot press is known (Patent Document 4). In order to maintain the transparency of a bisphenol A-type epoxy resin to be mixed with the microfibrillated cellulose, the microfibrillated cellulose is prepared by subjecting cotton linters to microfabrication about 20 times using a microfabrication apparatus such as a high-pressure homogenizer and an ultra-high pressure homogenizer.
On the other hand, Yano et al., who are inventors of the present invention, have produced a composite by laminating sheets prepared from a cellulose microfibril suspension, and impregnating them with a phenolic resin, followed by heating and pressurization in a mold (Patent Document 5).
Further, for the purpose of obtaining highly transparent members, Yano et al. have produced (1) resin composite sheets by impregnating sheet-like materials of microfibrillated cellulose derived from bacterial cellulose (BC) with a phenolic resin, followed by air drying for several hours, and laminating a required number of the sheets, followed by heat curing by hot pressing (Patent Documents 6 to 8). Moreover, Yano et al. have produced (2) resin composite sheets by impregnating sheet-like materials of BC-derived microfibrillated cellulose or pulp-derived microfibrillated cellulose with a monomer liquid of UV-curable acrylic resin (TCDDMA), followed by UV curing (Patent Documents 9 to 11). Furthermore, Yano et al. have produced (3) a composite resin composition comprising a bisphenol A-type epoxy resin and BC-derived microfibrillated cellulose or pulp-derived microfibrillated cellulose, the composition being cured for use as an adhesive or a sealant (Patent Document 12).
All of these documents intend to obtain highly transparent resin composite materials. Therefore, they are characterized by using BC originally having a small fiber diameter, or fibrillated cellulose having a nano-order fiber diameter and having a high specific surface area obtained by treating pulp with a high-pressure homogenizer, and then with a grinder 30 times.
Additionally, Yano et al. have produced a resin composite sheet by impregnating a sheet-like material of microfibrillated cellulose obtained by delignification/dehemicellulose in an aqueous potassium hydroxide solution with a monomer liquid of UV-curable acrylic resin (TCDDMA), followed by UV cross-linking (Patent Document 13).
On the other hand, unsaturated polyester resin molding materials obtained by adding fillers, curing agents, mold lubricants, pigments, thickeners, etc., to unsaturated polyester resins to obtain resin compositions; impregnating the resin compositions with reinforcing fiber substances, such as glass fiber; and forming the compositions into sheet- or bulk-like compositions are called sheet molding compounds (SMC), bulk molding compounds (BMC), etc., which are widely used in housing equipment, industrial components, and automobile components mainly after compression molding.
These molding materials are generally subjected to compression molding under heating. However, compression molding of larger-size products or more various kinds of products has drawbacks in that the cost burden, including securing of large-size molding machines, expensive mold investment, etc., is significantly increased. Generally, the compression molding temperature is about 120 to 160° C., and the compression molding pressure is as high as about 8 to 10 MPa. If compression molding can be carried out at low temperature and low pressure, the above cost burden can be reduced. However, compression molding at low temperature and low pressure results in underfills, and causes drawbacks in that cavities and pinholes are easily formed on the surface of the resulting molded products. Cavities and pinholes are undesirable because they impair the appearance of the molded products, and also have adverse effects on the mechanical strength of the molded products.
Moreover, microfibrillated plant fibers are known to have mechanical strength after being mixed with a phenolic resin or an epoxy resin. However, high molding pressure (several tens of MPa) was generally necessary to mold phenolic resin molding materials and epoxy resin molding materials comprising microfibrillated plant fibers. Furthermore, there was a problem that when two or more sheet-like resin materials comprising microfibrillated plant fibers were laminated and compressed into one molded article, the sheet-like resin materials were not integrated into one molded article at low pressure.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-312281    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-312688    Patent Document 3: Japanese Unexamined Patent Publication No. 9-124950    Patent Document 4: Japanese Unexamined Patent Publication No. 2006-316253    Patent Document 5: Japanese Unexamined Patent Publication No. 2003-201695    Patent Document 6: Japanese Unexamined Patent Publication No. 2005-60680    Patent Document 7: Japanese Unexamined Patent Publication No. 2006-35647    Patent Document 8: Japanese Unexamined Patent Publication No. 2006-36926    Patent Document 9: Japanese Unexamined Patent Publication No. 2006-240295    Patent Document 10: Japanese Unexamined Patent Publication No. 2006-241450    Patent Document 11: Japanese Unexamined Patent Publication No. 2007-51266    Patent Document 12: Japanese Unexamined Patent Publication No. 2007-146143    Patent Document 13: Japanese Unexamined Patent Publication No. 2008-24788    Patent Document 14: Japanese Unexamined Patent Publication No. 2005-42283    Patent Document 15: Japanese Unexamined Patent Publication No. 2008-13621